The present invention relates generally to cytokine receptors, and more specifically, to Type II (B Cell) Interleukin-1 receptors.
Interleukin-1.alpha. (IL-1.alpha.) and Interleukin-1.beta. (IL-1.beta.) are distantly related polypeptide hormones which play a central role in the regulation of immune and inflammatory responses. These two proteins act on a variety of cell types and have multiple biological activities. The diversity of biological activity ascribed to IL-1.alpha. and IL-1.beta. is mediated by specific plasma membrane receptors which bind both IL-1.alpha. and IL-1.beta.. Due to the wide range of biological activities mediated by IL-1.alpha. and IL-1.beta. it was originally believed that the IL-1 receptors should be highly conserved in a variety of species and expressed on a large variety of cells.
Structural chracterization by ligand affinity cross-linking techniques has demonstrated that, despite their significant divergence in sequence, IL-1.alpha. and IL-1.beta. bind to the same cell surface receptor molecule on T cells and fibroblasts (Dower et al., Nature (London) 324:266, 1986; Bird et al., Nature (London) 324:263, 1986; Dower ct al., Proc. Natl. Acad. Sci. USA 83:1060, 1986). The IL-1 receptor on murine and human T cells has been identified by cDNA expression cloning and N-terminal sequence analysis as an integral membrane glycoprotein that binds IL-1.alpha. and IL-1.beta. and has a molecular weight of 80,000 kDa (Sims et al., Science 241:585, 1988; Sims et al., Proc. Natl. Acad. Sci. USA 86:8946, 1989).
Subsequent affinity cross-linking studies indicate that IL-1 receptors on the Epstein Barr virus (EBV)-transformed human B cell lines VT)S-O and 3B6, the EBV-positive Burkitt's lymphoma cell line Rail, and the murine pre-B celt line 70Z/3, have a molecular weight of 60,000 to 68,000 kDa (Matsushima et al., J. Immunol. 136:4496, 1986; Bensimon et al., J. Immunol. 142:2290, 1989; Bensimon et al., J. Immunol. 143:1168, 1989; Horuk et al., J. Biol. Chem. 262:16275, 1987; Chizzonite et al., Proc. Natl. Acad. Sci. USA 86:8029, 1989; Bomsztyk et al., Proc. Natl. Acad. Sci. USA 86:8034, 1989). Moreover, comparison of the biochemical properties and kinetic analysis of the IL-1 receptor in the Raji B cell line with EL-4 murine T lymphoma cell line showed that Raji cells had lower binding affinity but much higher receptor density per cell than a subclone of EL-4 T cells (Horuk et al., J. Biol. Chem. 262:16275, 1987). Raji cells also failed to internalize IL-1 and demonstrated altered receptor binding affinities with IL-1 analogs. (Horuk et al., J. Biol. Chem. 262:16275, 1987). These data suggest that the IL-1 receptors expressed on B cells (referred to herein as type II IL-1 receptors) are different from IL-1 receptors detected on T cells and other cell types (referred to herein as type I IL-1 receptors).
In order to study the structural and biological characteristics of type II IL-1R and the role played by type II IL-1R in the responses of various cell populations to IL-1 stimulation, or to use type II IL-1R effectively in therapy, diagnosis, or assay, homogeneous compositions are needed. Such compositions are theoretically available via purification of receptors expressed by cultured cells, or by cloning and expression of genes encoding the receptors. Prior to the present invention, however, several obstacles prevented these goals from being achieved.
First, no cell lines have previously been known to express high levels of type II IL-1R constitutively and continuously, and cell lines known to express type II IL-1R did so only in low numbers (500 to 2,000 receptors/cell) which impeded efforts to purify receptors in amounts sufficient for obtaining amino acid sequence information or generating monoclonal antibodies. The low numbers of receptors has also precluded any practical translation assay-based method of cloning.
Second, the significant differences in DNA sequence between type I IL-1R and type II IL-1R has precluded cross-hybridization using a murine type IL-1R cDNA (Bomsztyk et al., Proc. Natl. Acad. Sci. USA 86:8034, 1989, and Chizzonite et al., Proc. Natl. Acad. Sci. USA 86:8029, 1989).
Third, even if a protein composition of sufficient purity could be obtained to permit N-terminal protein sequencing, the degeneracy of the genetic code may not permit one to define a suitable probe without considerable additional experimentation. Many iterative attempts may be required to define a probe having the requisite specificity to identify a hybridizing sequence in a cDNA library. Although direct expression cloning techniques avoid the need for repetitive screening using different probes of unknown specificity and have been useful in cloning other receptors (e.g., type I IL-1R), they are not sufficiently sensitive to be suitable for using in identifying type II IL-1R clones from cDNA libraries derived from cells expressing low numbers of type II IL-1R.
Thus, efforts to purify the type II IL-1R or to clone or express genes encoding type II IL-1R have been significantly impeded by lack of purified receptor, a suitable source of receptor mRNA, and by a sufficiently sensitive cloning technique.